(1) Field of the Invention
The present invention relates to an optical receiving apparatus and an optical receiving method for constituting an optical transmission system, and particularly, to an optical receiving apparatus and an optical receiving method for receiving and processing signal lights having different transmission rates.
(2) Prior Art
In a conventional optical transmission system, there have been adopted optical receiving apparatuses which are matched with the transmission rates of the signal lights, respectively. Concretely, there have been adopted dedicated optical receiving apparatuses which are adapted such as to 622 Mb/s, 2.5 Gb/s, and 10 Gb/s, respectively, which are the transmission rates in an SDH (Synchronous Digital Hierarchy) system.
In such a conventional optical receiving apparatus, each of the received signal lights is converted into an electric signal such as by means of a light receiving element, and linearly amplified such as by an equalizing amplifier. Then, such as discrimination processing is effected for the received data, making use of a clock signal which is regenerated based on the amplified signal. To accurately effect the reception processing, there have been separately established such as the equalizing band of the equalizing amplifier and the filter band of the clock signal regeneration system, corresponding to each of the transmission rates of the signal lights. Thus, it has been necessary to provide a dedicated optical receiving apparatus for each of the transmission rates of the signal lights.
Meanwhile, in a wavelength-division multiplexing (WDM) type of network in which a plurality of signal lights having different wavelengths are transmitted within a single optical fiber, the transmission rate of each of lights may differ from one another depending on the wavelength. Then, it is demanded to receive each of such signal lights having different transmission rates by an optical receiving apparatus, with flexibility. To realize that, it becomes necessary that the optical receiving apparatus itself is provided with a function to automatically detect the transmission rate and to switch its operation based on the detected transmission rate, differently from the conventional technique where the optical receiving apparatuses are exchanged for each of the transmission rates, respectively. In this case, to optimize the reception characteristics of the optical receiving apparatus, it is desired to optimally control such as the equalizing band of the equalizing amplifier and the filter band of the clock signal regeneration system, corresponding to the detected transmission rate.
Regarding an equalizing amplifier, there occurs such a problem when its equalizing band is fixed at constant, as follows. In case that a signal at a rate higher than the equalizing band is received, the waveform is distorted due to band limitation exerting onto the signal output from the equalizing amplifier, thereby deteriorating the reception characteristics. In another case that a signal at a rate lower than the equalizing band is received, there is increased a high-frequency noise component at the equalizing amplifier, thereby also deteriorating the reception characteristics. To avoid such deterioration of the reception characteristics, it is necessary to control the equalizing band corresponding to the transmission rate of the signal light.
However, the conventional optical receiving apparatuses do not have a function to optimally control such as the equalizing band, by detecting the transmission rate of the received signal. Thus, in case of applying the optical receiving apparatus to the network of the aforementioned wavelength-division multiplexing system, it is required to provide an optical receiving apparatus for each of the transmission rates, causing a problem that the constitution is complicated, with higher cost.
As a conventional technique for detecting the transmission rate of a received signal, there is known such as Japanese Unexamined Patent Publication No.5-102996. In this conventional technique, there is measured the number of shifting points of received data within a fixed period of time, and it is judged as to whether the measured number of the shifting points is in of a normal range or an abnormal range, to thereby detect a change of the transmission rate of received signal. If this technique is applied to the aforementioned conventional optical receiving apparatus, it becomes possible to detect the change of the transmission rate of the received signal light. However, there cannot be obtained optimal reception characteristics corresponding to the transmission rate. Further, in case of detecting the transmission rate making use of an output such as of the equalizing amplifier of the conventional optical receiving apparatus, there exists such a problem that the change of the transmission rate cannot be detected accurately when a signal light at a rate higher than the equalizing band is received.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide an optical receiving apparatus and an optical receiving method in which signal lights having different transmission rates can be assuredly received and processed with a simple constitution.
Thus, the optical receiving apparatus according to the present invention comprises: a light receiving element for receiving each of signal lights having different transmission rates and for converting the received signal light into an electric signal; an equalizing-amplifying device having a variable band, for linearly amplifying the electric signal from the light receiving element; a transmission rate detecting device for detecting a transmission rate of the received signal light; and an equalizing-amplification controlling device for controlling the band of the equalizing-amplifying device, corresponding to the transmission rate which is detected by the transmission rate detecting device.
According to such a constitution, the signal light received in the present optical receiving apparatus is firstly converted into an electrical signal by the light receiving element, and then sent to the equalizing-amplifying device where the converted signal is linearly amplified. At this time, the band of equalizing-amplifying device is controlled by the equalizing-amplification controlling device, corresponding to the transmission rate detected by the transmission rate detecting device, thereby avoiding deterioration of the reception characteristics even when the transmission rate of the received signal light is changed.
Therefore, it becomes possible to receive and process signal lights having different transmission rates by a single optical receiving apparatus, thereby dealing with such as wavelength-division multiplexing type of network with flexibility, while reducing cost of the optical transmission system.
Preferably, the equalizing-amplification controlling device comprises a device for controlling the band of the equalizing-amplifying device to be a maximum, when received a rate-change signal indicating that the transmission rate of the received signal light has been changed; and a device for, on and after the transmission rate after changing has been detected by the transmission rate detecting device, controlling the band of the equalizing-amplifying device to be an optimal value corresponding to the detected transmission rate.
Thus, the band of the equalizing-amplifying device is controlled to be a maximum at the time of change of transmission rate, so that the received signal is not distorted by band limitation of the equalizing-amplifying device even when the transmission rate is changed from a lower rate toward a higher rate. Further, the band of the equalizing-amplifying device is controlled to be an optimal value corresponding to the detected transmission rate when the transmission rate after changing is detected by the transmission rate detecting device, thereby obtaining excellent reception characteristics. Therefore, the signal lights having different transmission rates can be received and processed with accuracy.
The aforementioned optical receiving apparatus may be constituted to further comprise a clock regenerating device for regenerating a clock signal based on a signal from the equalizing-amplifying device, making use of a phase-locked loop circuit having a loop filter having a variable band; and a clock regeneration controlling device for controlling a band of the loop filter corresponding to the transmission rate detected by the transmission rate detecting device.
According to such a constitution, there is regenerated a clock signal by the clock regenerating device making use of a phase-locked loop circuit, based on a signal from the equalizing-amplifying device. At this time, the band of loop filter constituting the phase-locked loop circuit is controlled by the clock regeneration controlling device corresponding to the transmission rate detected by the transmission rate detecting device, so that a stable clock signal can be obtained even when the transmission rate of the received signal is shifted.
Preferably, the clock regeneration controlling device comprises a device for controlling the band of the loop filter to be a maximum, when received a rate-change signal indicating that the transmission rate of the received signal light has been changed; and a device for, on and after the transmission rate after changing has been detected by the transmission rate detecting device, controlling the band of the loop filter to be an optimal value corresponding to the detected transmission rate.
Thus, the band of the loop filter is controlled to be a maximum at the time of change of the transmission rate, so that the pull-in time of the phase-locked loop circuit is shortened. Further, when the transmission rate after changing is detected by the transmission rate detecting device, the band of the loop filter is optimized corresponding to the thus detected transmission rate, so that Q of the clock signal is kept substantially constant thereby regenerating a stable clock signal.
Further, the phase-locked loop circuit may include a voltage controlled oscillating part which is capable of generating oscillation signals at frequencies corresponding to different transmission rates, respectively, and the clock regeneration controlling device may comprise a device for controlling the oscillation frequency of the voltage controlled oscillating part, in a manner corresponding to the transmission rate detected by the transmission rate detecting device.
Thus, there can be output, from the voltage controlled oscillating part, a clock signal corresponding to each of the different transmission rates, so that the constitution of the clock regenerating device can be simplified.
In addition, the rate-change signal can be generated based on either of: halting of the reception of signal light; change of the output of the transmission rate detecting device; or a code error or outing of synchronism in a discrimination processing of the received signal.
As a concrete constitution of the transmission rate detecting device, it is preferable that the transmission rate detecting device comprises a filtering part for receiving, as an input signal, a signal from the equalizing-amplifying device, the filtering part having a band narrower than the minimum band of the equalizing-amplifying device, a rectifying part for rectifying a signal passed through the filtering part, and an averaging part for obtaining an averaged value of level of a signal rectified by the rectifying part, and for detecting a transmission rate of the received signal light, based on the averaged value.
According to such a constitution, it becomes possible to assuredly detect the transmission rate without being affected by the band change of the equalizing-amplifying device, even when the transmission rate detecting device detects the transmission rate based on the signal from the equalizing-amplifying device.
An optical receiving method according to the present invention comprises the steps of: receiving each of signal light having different transmission rates and converting the received signal light into an electric signal; detecting a transmission rate of the received signal light; and equalizing-amplification-controlling a variable band of equalizing-amplifying device which linearly amplifies the converted electric signal. Further, the equalizing-amplification-controlling step preferably comprises the steps of: controlling the band of the equalizing-amplifying device to be a maximum, when the transmission rate of the received signal light has been changed; and, on and after the transmission rate after changing has been detected, controlling the band of the equalizing-amplifying device to be an optimal value corresponding to the detected transmission rate.
The optical receiving method may further comprise the steps of: regenerating a clock signal based on the signal linearly amplified by the equalizing-amplifying device, making use of a phase-locked loop circuit having a loop filter having a variable band; and controlling a band of the loop filter corresponding to the detected transmission rate.
Further objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.